(1) Field of the Invention
The present invention relates to a state-of-charge adjusting apparatus, in particular, a state-of-charge adjusting apparatus, which includes: a voltage detecting means for detecting each voltage of a plurality of unit cells, the plurality of the unit cells consisting of secondary batteries connected in series; a discharge resistance for discharging the unit cell; and a capacity equalizing means for controlling discharge of the plurality of the unit cells through the discharge resistance on the basis of a voltage of each unit cell detected by the voltage detecting means so that the voltage values of the plurality of the unit cells become uniform.
(2) Description of the Related Art
Recently, a hybrid electric-internal combustion vehicle (hereinafter, HEV) which travels by using both of an engine and an electric motor has been widely spread. The hybrid motor vehicle includes two types of battery consisting of: a low-voltage battery having a voltage of about 12 V for starting the engine; and a high-voltage battery for driving the electric motor. The high-voltage battery provides a high voltage by connecting a plurality of unit cells in series, the unit cell being a secondary battery such as a nickel-hydrogen battery or a lithium battery.
As for the high-voltage battery, dispersion (i.e. non-uniformity) of a voltage of each unit cell, that is, dispersion of a state-of-charge (SOC) occurs by repeating charge and discharge. Therefore, upon charge and discharge of the battery, from the viewpoint of durability and safety of each cell, it is necessary to prohibit a charge at a time point when a voltage of a unit cell having the highest SOC (or the highest voltage) reaches a predetermined upper limit SOC (or a predetermined upper limit voltage value) and at a time point when a voltage of a unit cell having the lowest SOC (or the lowest voltage) reaches a predetermined lower limit SOC (or a predetermined lower limit voltage value).
Therefore, when dispersion of SOC occurs for each unit cell, a usable capacity of the battery is substantially decreased. As for an HEV, this undesirable phenomenon makes so-called an assist and regeneration insufficient, wherein the assist means a supplement of battery energy with respect to gasoline upon hill-climbing and the regeneration means regeneration of energy to the battery upon hill-coming down, thereby causing deterioration in a vehicle dynamic characteristic or fuel efficiency.
In order to solve the above problems, proposed is a discharge-type state-of-charge adjusting apparatus, in which each voltage of a plurality of unit cells are detected so as to calculate an average voltage value of the detected voltage values, the unit cells each having the voltage value higher than the calculated average voltage value are connected to the respective discharge resistances, and stored charge of each connected unit cell is subjected to discharge until the voltage of the connected unit cell reaches the average voltage value, said steps described above being repeated, so that dispersion (i.e. non-uniformity) of the voltage of each unit cell is removed (for example, see Japanese Patent Application Laid-Open No. 2002-369400).
However, in the conventional state-of-charge adjusting apparatus as described above, each unit cell is in turn connected to the discharge resistance and subjected to discharge one by one, therefore there is a problem that it takes so much time to reach the capacity equalization.